Adding details to low poly model for 3d printing
Hi,
if you need more details, you will have to add more vertices. You can reach it by several ways. 

Globally via mesh modifiers(you can find it in context menu, which is defaultly on right side): Subdivision(Don't forget to use "Simple" option) or Multiresolution.

For specific parts: Edit mode -> Select vertices/lines/faces -> Right click -> Subdivide - You can also change some parameters(smoothness, number of cuts, ...)

There's also dyntopo - it's tool for sculpting, but it can cause unwanted results for 3d printing.

Don't forget some too detailed parts would be better to separate to more prints.

I hope this is that what you needed to know.